Sudden infant death syndrome (SIDS) is the most common cause of post neonatal infant death in the U.S. The mechanisms are unknown, but abnormalities in the serotonin system have been consistently found in autopsy studies. Thus, defining the role of serotonin neurons in normal brain function, which is also an important scientific goal, may help to understand SIDS. We have proposed that serotonin neurons of the raphe nuclei are sensors of arterial carbon dioxide that respond to hypercapnia by inducing arousal, an increase in breathing, and other changes aimed at restoring pH/CO2 homeostasis. This is consistent with the epidemiology of SIDS, because a defect in CO2 chemoreception could lead to loss of the normal ventilatory response to airway obstruction or rebreathing during sleep. We now plan to use two lines of genetically altered mice in which there is a defect in development of serotonin neurons to directly test the hypothesis that serotonin neurons are CO2 chemoreceptors. One line of mouse lacks Pet1, and has only 30% of the normal number of serotonin neurons. Our preliminary data indicate that male Pet1 KO mice have a defect in the hypercapnic ventilatory response as adults. The other line of mouse is a tissue specific knockout of Lmx1 b in cells that co-express Pet1. These mice have a selective loss of >99% of serotonin neurons. They live to adulthood with relatively few abnormalities. However, they have a severe defect in CO2 chemoreception. We will expose adult and neonatal mice to hypercapnia and hypoxia, and measure ventilation, heart rate, motor activity and EEG. We will also examine the effect on wild-type and mutant mice of prenatal exposure to cigarette smoke. We will then use the perfused brain preparation and brain slices to determine whether there is an alteration of respiratory rhythm generation or decreased response of the respiratory network to CO2. Finally, using in vivo, perfused brain and brain slice preparations, we will determine whether serotonin or thyrotropin releasing hormone can restore normal breathing in adult Lmxlb conditional knockout mice. These experiments will help define the normal role of serotonin neurons in control of breathing, autonomic function and arousal, and may explain how a defect in the serotonin system could lead to death while sleeping during a critical developmental period. This may provide a specific neurobiological mechanism for the association between SIDS and defects in the serotonin system.